Speedometer Cable Adapter question

[53Rick]

I have a '72 GMC 1/2-ton with a Turbo 400 transmission. Recently the speedometer stopped working. I checked cable connections to be sure they were tight. Just starting to troubleshoot it. Crawling underneath I noticed the cable is tight where it connects to the speedometer cable adapter. However I noticed there is some wiggle in the adapter and it rotates somewhere close to 180 degrees when I turn it. I don't know what this adapter does and before diagnosing further, I wanted to be sure that the adapter is supposed to turn freely. If so, I will move on. If not then any help on determining why it rotates would be appreciated. I have read the shop manual and the overhaul manual and can find nothing that speaks to the adapter. The adapter I am talking about is called an adapter in the manual and the speedometer cable connects to it at the transmission. Any assistance appreciated.